The goal of the invention is simplification and acceleration of the drill-out of rock cores and of reducing the costs for extraction of rock core samples.
The invention makes it possible to obtain rock core samples form a significant formation depth, without having to postdrill the primary drill hole after each drill-out of a rock core. A core hole with its usually much smaller diameter can be drilled beyond the bottom of the primary drilled hole to a length of, for example, one hundred or more meters, without any intermediate postdrilling. In cases where the drilling is stopped due to information obtained from the rock samples, postdrilling is unnecessary, so that considerable cost savings will results. At the same time, the drilling time is reduced, even when postdrilling is needed when continuing the drilling project, because postdrilling along a distance corresponding to a multiple of one core length is faster to complete than a number of separate postdrilling processes along drill routes of single core lengths. The type of core drill hole resulting from the present invention has a continuous profile, because the core drill holes are cut under constant conditions due to the unchanged position of the outer tube of the drilling tool bracing the coring device. The outer tube of the drilling tool forms a type of underground replacement for a stopped turntable.
The insertion into the coring unit of an extender when the coring unit is pulled out of the hole after each core extraction is exceptionally simple and quick, and upon removal of the sequentially inserted extenders after completion of one core drilling operation and before postdrilling of the primary hole, groups of extenders can be removed and unscrewed from each other, regardless of the coring unit being used. The extraction of rock core samples according to the invention makes it possible, when postdrilling the primary hole, to use not only the usual roller cone type core bit, but also to use a fixed-cutter type drag bit as a part of the drilling tool. The latter type of bit combines greater drilling performance with far greater strength and is able to avoid every second round trip needed for postdrilling when using core bits.